The invention pertains to a device for the cleaning of flue gas and more particularly, to such a device which prevents material deposits therein.
For cleaning of flue gas, for example, for cleaning of exhaust gases from incineration systems, at present mostly wet methods, semidry methods and dry methods are being used. In this case, wet methods and semidry methods, as a rule, have higher cleaning efficiency than dry methods. However, wet or semidry methods require a greater expense for the inlet, outlet and reprocessing of the adsorbant/absorbant employed for cleaning of the raw gas. In addition, in the container in which the added adsorbant/absorbant causes the absorption or adsorption of the pollutants, there are extremely aggressive acids which attack the inside wall of the container or built-in parts located therein. The waters ejected with salts from the scrubber, must be treated in an effluent processing system, which is associated with a considerable expense which has an effect, in particular, on smaller flue gas cleaning systems.
Therefore, various methods were proposed to improve the cleaning efficiency of dry flue gas cleaning methods. For example, it is disclosed in EP-A-0 104 335 that the reaction between the pollutants contained in the raw gas and the adsorbant/absorbant can be improved when additional water is added. This takes place in such a manner that in a first step, dry, powdery reaction agent is blown in and in a second step, pure water or an aqueous solution or suspension or a reaction medium is sprayed in. Due to the water added in the second step, a reactivation of particles of the reaction medium can be achieved. Due to the addition of water, the reaction medium can be used more efficiently.
The disadvantage in this case is that the added quantity of water must be kept as small as possible so that even at relatively low initial of the exhaust, the dew point temperature will be maintained. If this is not observed, then there will be damage to the system due to the forming of aggressive, aqueous acids.
Furthermore from EP-A-0 029 564 it is known that due to an intensive mixing of the raw gas with the added, dry adsorbant/absorbant, an increase in the relative speed between the gas and the adsorbant/absorbant particles, and thus an improvement in the reaction rate or of the cleaning efficiency can be achieved. In this regard it is proposed in EP-A-0 029 564 to allow sound to influence the reactants.
From DE-A-32 32 080 it is known that through partial, external return of the adsorbant/absorbant or of the deposited solids, an improvement in utilization of the adsorbant/absorbant and thus ultimately a savings in adsorbant/absorbant can be achieved.
To improve the cleaning efficiency, in EP-B-0 203 430 a method or a device for cleaning of flue gas is proposed, whereby the ratio of the dwell time of an adsorbant/absorbant supplied to the reactor, to the dwell time of the flue gas in the reactor, is controlled and/or regulated in that built-in parts are provided in the reactor which move with respect to the reactor container, for example, in the form of a rotating worm. This worm can be driven at a speed of about 0.5-120 rpm, whereby the adsorbant/absorbant is stored partly on the surface of the worm gear; however, it is constantly swirled up by the flue gases to be cleaned, or by additionally injected compressed air. Thus the dwell time of the heavier adsorbant/absorbant particles can be controlled or regulated relative to the dwell time of the gas in the reactor.
However, the disadvantage of this device or of this method, is that with increasing operating time of the reactor, a hard layer forms on both the internal wall of the container and also on the surface of the moving built-in parts; this layer forms due to the solids located in the reactor. This extremely hard and tough layer can practically only be removed by using mining tools.
Operating experiences with the aforementioned devices or methods have shown that a uniform atomizing pattern of the sprayed water and a uniform distribution of the solids with respect to the flue gas quantity cannot be attained. Due to the resultant irregular and ineffective mixing of flue gas and adsorbant/absorbant, it is unavoidable that wet zones lead to baked-on deposits and dry zones have unsatisfactory absorption.